Question: Which of these numbers is prime? ${25,\ 31,\ 44,\ 49,\ 95}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 25 are 1, 5, and 25. The factors of 31 are 1 and 31. The factors of 44 are 1, 2, 4, 11, 22, and 44. The factors of 49 are 1, 7, and 49. The factors of 95 are 1, 5, 19, and 95. Thus, 31 is a prime number.